1. FIELD OF THE INVENTION
The invention relates generally to scanners and scanning methods, and more particularly to those having or using a scan bar.
2. DESCRIPTION OF THE RELATED ART
A typical stand-alone scanner or multifunction scanner/printer uses a scan bar to acquire image data. The scan bar generates an analog signal that represents the image data. The scanner then converts the analog signal into its digital equivalent using an analog-front-end (“AFE”) circuit. Typically, the AFE is an analog integrated circuit that samples the analog signal from the scan bar and converts the sampled analog signal into digital image data for subsequent use by a computer or a processor.
AFE circuits have limitations with respect to the signals they can convert to digital form. For example, most circuits are limited to processing input signals that fall within a certain range, for example, a range of 0V to 3V. High quality scans can be achieved when the AFE is able to use this entire range for analog to digital conversion. However, scan bar outputs rarely match the range of available AFEs. Therefore, adjustments must be made to accommodate this mismatch. Most often, the analog signal from the scan bar is modified with offset and gain adjustments. In some cases, offset and gain parameters may be programmed into the AFE. The offset and gain are applied to the analog signal via respective subtraction and multiplication processes. Determining the offset and gain generally involves scanning a set of target image data (or target) under different conditions. For example, the target is first scanned with no illumination (a “dark” condition scan), followed by a scan with a predetermined level of illumination (a “white” condition scan). Typically, the predetermined level of illumination is obtained from some calibration parameters of an illumination source, and the target is a white calibration strip.
Once target image data has been acquired under the “dark” and “white” conditions, the AFE can be calibrated by different techniques. For example, a minimum point from the “dark” condition scan is initially searched and obtained, and set to be the offset. Similarly, a maximum point from the “white” scan is searched and obtained. The offset is subsequently subtracted from the maximum point. An upper limit of the dynamic range of the AFE is divided by the difference between the offset and maximum point to yield a quotient. The quotient is used as the gain.
Another calibration technique requires determining an average or a rolling or moving average from the set of target data during the dark condition scan. (A rolling or moving average is generally obtained from averaging a weighted subset or window of the target data such that the average is biased towards a maximum or minimum.) Subsequently, a set of predetermined “dark” and “white” target values are determined. The “dark” target value is subtracted from the average to obtain another difference. This difference is used as the offset. Similarly, a second average or a second rolling or moving average from the set of target data during the white condition scan is obtained. The offset is then subtracted from the second average to obtain a third difference. The “white” target value is divided by the third difference and the result or quotient is used as the gain.